The present invention relates generally to an operational control device, and more particularly to a pump system and a method for operating a pump system.
Certain machine tools must be charged with coolant and/or lubricant at operating pressures which can reach 25 bar and more. As such, pumps used to effect such charging can be particularly important. In connection with industrial drilling, milling or tapping processes, and with fluid charging at the pressures mentioned, high cooling performance and correspondingly high process speeds are desirable.
For high pressure coolant supply, positive displacement pumps have been used in the machine tool sector based on their ability to provide, in a single compact unit, fluid at pressures which can reach 80 bar. Such pumps, understandably, have advantages over conventional centrifugal pumps for high-pressure applications.
In some applications, screw pumps, and three-screw pumps in particular, have been used as positive displacement pumps. Such screw pumps have low-pulsation and even delivery characteristics. They also have high wear resistance.
Due to their design, however, screw pumps (like other positive displacement pumps) require the use of a pressure regulating valve in order to keep delivery pressure constant. Such a pressure regulating valve may be provided in a system along with the associated machine tool. Screw pumps are operated with a constant rotational speed and, due to the positive displacement characteristics thereof, provide an approximately constant delivery. Since the machine tool being serviced by the pump often requires fluid delivery at a volume that is less than the flow volume provided by the pump, the excess delivery (referred to as differential delivery) is discharged through the pressure regulating valve. One result of this arrangement is that the efficiency of the system, as compared to the often high efficiency of the positive displacement pump, is reduced because a portion of the pump output necessary for the pressure build-up in the differential delivery is not used.
In the event of breaks in operation (e.g., when changing tools or the like), coolant lubricant is not pumped to the machine tool. To accommodate this, either a shut-off valve is installed in the supply line for the machine tool, or the pump is switched off. Due to the high mechanical load involved, switching off is usually only worth considering in the case of systems which operate at relatively low pressure. In systems with a shut-off valve, the pump continues to operate (i.e., with the shut-off valve closed) with the full pump discharge being accommodated by the pressure regulation valve. Such an arrangement understandably has a disadvantageous effect on system efficiency. In order to reduce the power requirement during breaks in operation, a controllable pressure regulating valve that can be depressurized during the breaks in operation, is often used.
The use of pressure regulating valves having variable pressure capabilities is known. Such valves have the advantage that the fluid supply can be adapted to the requirements of the process in a suitable manner. For example, in the case of tools having a low pressure requirement, the power consumption of the positive displacement pump falls with the pressure. Even so, for cases in which a valve is used, the power consumption of the pump is usually higher than the actual power requirement for the fluid supply to the tool, since a higher delivery is provided by the pump than is required by the tool. As coolant supply and cooling account for up to 35% of the energy consumption of a machine tool, the potential for improvement/optimization is considerable.
Using valves for pressure control includes additional disadvantages. For example, in systems used to supply coolant lubricant to machine tools, the switching of the valve(s) causes pressure pulsations which can heavily load the system and can even cause mechanical damage to system and tool components.
An alternative approach involves varying the rotational speed of the pump motor by means of a frequency converter. In such cases, system pressure downstream of the pump is monitored (e.g., using a pressure sensor) and is passed to a frequency converter as a closed-loop control variable. In this way the pump motor rotational speed is controlled as an open-loop control variable by means of a PI (proportional-integral) closed-loop control by means of the frequency converter.
Such a closed-loop control of this type—using a classic closed-loop control method—has the disadvantage, however, of insufficient dynamic response. In particular, it is not possible to obtain a rapid run up of the pump motor to its setpoint rotational speed, or to the setpoint pressure, without causing disadvantageous overshoot. By contrast, a more strongly damped rise leads to comparatively long run up and attendant response times, which, in turn, disadvantageously results in unproductive idle times of the associated machine tool or the like. In some applications, it has proven desirable to reach a setpoint value in no more than 500 milliseconds (ms) following switching on. Such a requirement, however, cannot be achieved in practice with known closed-loop control algorithms in the present context of the operational control of a screw pump.
Combinations of the previously described approaches have also been attempted. Thus, a closed-loop control of the pump motor using pump discharge pressure as open-loop control variable is combined with a downstream pressure regulating valve of the type previously described. Such approaches, however, require disadvantageously high outlays of equipment and/or result in poor dynamics.
An example of an operation control device for a positive displacement pump with a pump motor is disclosed in US Patent Application Publication No. 2002/0094910. Actuation means for rotational speed actuation for a pump motor are provided, along with state sensor means to detect a current operating parameter of the positive displacement pump in the form of an oil temperature. Operating mode means for predetermining an operating mode of the positive displacement pump are connected upstream of the actuation means.